1. Field of the Invention
This invention relates to acoustic diaphragms for loudspeakers and more particularly, to materials useful for making the diaphragms of the just-mentioned type.
2. Description of the Prior Art
Hitherto, diaphrams for loudspeakers have been made of a wide variety of materials. In U.S. Pat. No. 4,190,746, a number of plastics materials are enumerated including, for example, polystyrene, polyvinyl chloride, polymethacrylamide, cellulose acetate, acrylic resins, polyacrylonitrile resin, polyacrylamide, phenolic resins, unsaturated polyester resins, polyoxy resins, polyurethane resins, olefins resins and the like. In this patent, it is disclosed that polypropylene homopolymer or copolymers with minor proportions of olefinically unsaturated copolymerizable monomers such as ethylene can provide a diaphragm for a loudspeaker which can be used satisfactorily over a wide frequency range.
An acoustic diaphragm which is formed of polypropylene is also disclosed in Japanese Patent Publication No. 55-46112. In Japanese Kokai Publication No. 53-45226, there is disclosed a diaphragm formed of 4-methylpentene-1 resin.
However, these known synthetic plastic resin diaphragms have the drawbacks that though their internal loss is great, the propagation velocity and mechanical strengths are so low or small that loudspeakers using these acoustic diaphragms do not exhibit smooth acoustic or frequency chacteristics.